It has been known for long to produce dental restorations by means of positive models which are cast with gypsum. After curing, the positive model is removed, usually by melting out. In traditional technology, it consists of wax such that the term “lost wax technology” has become established for the entire process.
In order to be able to produce several dental restoration models in one go several positive models are also required which are connected to one another via so-called sprue channels or material feeders. With the finished negative mold, the cavities for the dental restorations can be filled together and in one go.
Basically, processes of this type are also usual for the casting of dental restoration parts. In the pressing technology, it needs to be taken into account additionally that the produced muffle of gypsum is sufficiently strong to prevent so-called muffle cracks which, as a rule, result in a loss of the entire dental restoration which has been produced. Here, the wall thickness between the outer edge of the muffle and the closest adjacent position of the positive model or the related mold cavity is particularly relevant. Said wall thickness should not fall below a certain degree, for instance three centimeters.
On the other hand, it is desired to produce as many dental restorations as possible simultaneously such that particular effort is made to distribute the dental restoration in the manner of a wax tree by means of a corresponding number and arrangement of material feeders or sprue channels, based on a so-called muffle mandrel whose shape corresponds to the resulting press channel.
By now, it has also been known for a long time to produce the positive model by rapid prototyping and to use the model produced in this way as a basis for the molding process. An example of a solution of this type may be taken from WO/95/28688.
Also, it has already been suggested to optimize the exact position of the material feeder, its angle relative to the positive model and the so-called sprue region, that is to say the region at which material feeder and positive model meet, by software. In this connection, a good solution may be taken from the European patent application 14 171 268.7. Typically, the so-called sprueing process takes place obliquely from the side in the region of the occlusal or rather incisal side of the dental restoration.
However, it has also been known for a longer time to use green products with color gradients and to impart a corresponding color gradient to the tooth which is to be produced.
An example of this is the solution known from EP 2 065 012 A1, which already represents an improvement of the original so-called multi press process which has also been developed by the present applicant.
Besides the production by means of rapid prototyping it is also possible to produce the positive model with the related material feeder by milling by means of a CNC machine. Typically, this works for not too complex and contorted structures of the wax tree to be produced in this manner, as above a certain degree of complexity the milling cutter—even in case of five-axis milling machines—cannot reach the proper position to achieve the desired milling result.
Typically, green products of this type are produced from specific polymers which can be milled easily, but which can also be burnt out easily and which are milled from a solid block, in most cases from a disc having a diameter or 98.5 mm which is prepared for a standard milling reception.
Discs of this type can, for instance, have a thickness of 3 cm such that the integrated production of a wax tree having a corresponding height would also be possible if the above-mentioned problem did not exist.
Typically, the positive model is milled together with its related material feeder or sprue channel, readily in combination with one or several further positive models.
In this connection, comparatively much material is lost as the reproduction of the wax tree occupies a corresponding amount of space of the green product disk.
In the realization of mold cavities for dental restoration using the lost wax technology, another problem is that the muffles used for this purpose are surprisingly prone to muffle cracks. Typically, the lost material has a larger coefficient of thermal expansion than the surrounding gypsum material.
In case the heating does not take place exactly according to instruction, a plug of material can arise which initially obstructs the flow of wax to the outside and which leads to such a high positive pressure in the mold cavity due to the different coefficients of thermal expansion that the formation of cracks arises.